Massive stars eventually collapse under their own weight and explode as supernovae. Smaller stars, like our sun, simply fizzle into so-called white dwarfs. Scientists are keen to know just where the cutoff point is, not least because supernova explosions are thought to create most of the universe's heavy elements--the building blocks of dust, planets, and people.

Now a team of Cambridge University astronomers says it has found a white dwarf that must have had a huge progenitor, suggesting that it may take more mass than some had thought for stars to culminate in supernovae. "This is pretty exciting," says astronomer Mike Bolte of the University of California, Santa Cruz. Scientists have assumed the "mass cutoff" point to lie between five and 10 times the mass of our sun. Now that window of uncertainty has been narrowed.

The astronomers, led by Rebecca Elson, located their dwarf while using the Hubble Space Telescope to investigate star formation in a cluster (named NGC 1818) in the Large Magellanic Cloud, a small neighboring galaxy. They already knew that the cluster's stars were formed about 40 million years ago. Only a massive star would burn out within that time period. Moreover, the burnout was recent, because the dwarf was not only blue and therefore very hot, but also very bright.

Knowing its age, the team was able to estimate that the dwarf is the remains of a star of about 7.6 solar masses, says team member Steinn Sigurdsson. And that, he says, significantly hikes up the lower limit of a "very critical boundary." Their report will appear in a forthcoming issue of Astrophysical Journal Letters.

The new observation adds to knowledge about supernovae, the fertilizers of the universe. "Just when you stop making supernovae and start making white dwarfs determines the whole chemical constitution of the universe," says Sigurdsson. A more precise supernova threshold could also hold clues to the rate at which massive stars were born earlier in cosmic history, he adds. "If you really need massive stars [to account for all the heavy elements around] and there aren't many now, there have to have been more in the past."